FIG. 51 is a circuit diagram showing the construction of a battery pack according to background art. A battery pack 1001 shown in FIG. 51 is provided with a protection circuit 1002 and a secondary battery 1003. The secondary battery 1003 is, for example, a rechargeable secondary battery such as a lithium ion secondary battery, a lithium polymer secondary battery, a nickel-metal-hydride secondary battery or a nickel-cadmium secondary battery. In the case of overcharging or excessive discharge current, such a secondary battery might have characteristics thereof such as cycle life degraded or might undergo expansion, deformation and the like. Accordingly, the battery pack 1001 is provided with the protection circuit 1002 for protecting the secondary battery 1003 from overcharging or excessive discharge current (see, for example, patent documents 1, 2).
The protection circuit 1002 includes external connection terminals 1004, 1005, FETs (Field Effect Transistor) 1006, 1007, reference voltage generators 1008, 1009, comparators 1010, 1111, a resistor 1112 and a logic circuit 1013.
The external connection terminals 1004, 1005 are connection terminals for connecting a charger for charging the secondary battery 1003 and for connecting a power source for driving a mobile device such as a mobile phone or a digital camera, an electric tool, a robot or an electric automobile that is driven by a discharge current from the secondary battery 1003. The external connection terminal 1004, the secondary battery 1003, the FET 1006, the FET 1007 and the external connection terminal 1005 are connected in series.
The FET 1006 is oriented such that an anode of a parasite diode is connected to the secondary battery 1003, whereas the FET 1007 is oriented such that an anode of a parasitic diode is connected to the external connection terminal 1005. The FET 1006 is used as a switch for protection from excessive discharge for cutting off a discharge current if the discharge current of the secondary battery 1003 becomes excessive, whereas the FET 1007 is used as a switch for protection from overcharging for cutting off a charge current if the secondary battery 1003 is overcharged.
Further, a positive electrode terminal of the secondary battery 1003 is connected to a plus terminal of the comparator 1010, a reference voltage Vref1 outputted from the reference voltage generator 1008 is applied to a minus terminal of the comparator 1010, and an output terminal of the comparator 1010 is connected to the logic circuit 1013. A voltage for detecting the overcharging of the secondary battery 1003 is set as the reference voltage Vref1. The comparator 1010 outputs a detection signal representing overcharging to the logic circuit 1013 if the secondary battery 1003 is charged by an unillustrated charger connected to the external connection terminals 1004, 1005 and a terminal voltage of the secondary battery 1003 exceeds the reference voltage Vref1.
Further, a connection point of the FETS 1006, 1007 is connected to a minus terminal of the comparator 1111 via the resistor 1112, and a reference voltage Vref2 outputted from the reference voltage generator 1009 is applied to a plus terminal of the comparator 1111. Thus, a voltage drop caused by the on-resistance of the FET 1006 resulting from the flow of a discharge current from the secondary battery 1003 is applied to the minus terminal of the comparator 1111 via the resistor 1112. The reference voltage Vref2 is set to a voltage corresponding to a voltage drop caused by the on-resistance of the FET 1006, for example, in the case where a maximum discharge current within such a range as not to cause the characteristic degradation of the secondary battery 1003 flows through the FET 1006.
The comparator 1111 detects an increase of the voltage drop in the FET 1006 and outputs a detection signal representing overcurrent discharge to the logic circuit 1013, for example, if the external connection terminals 1004, 1005 are short-circuited together due to the contact with a metal piece or the breakdown of a load device connected to the external connection terminals 1004, 1005 and an excessive discharge current flows from the secondary battery 1003.
The logic circuit 1013 turns the FET 1007 off to stop the charging of the secondary battery 1003 if a detection signal representing overcharging is outputted from the comparator 1010 while turning the FET 1006 off to stop the discharge of the secondary battery 1003 if a detection signal representing overcurrent discharge is outputted from the comparator 1111. In this way, the protection circuit 1002 protects the secondary battery 1003 from overcharging and overcurrent discharge.
Further, a known protection circuit for protecting a secondary battery from overcharging and overcurrent discharge in this way is such as in a battery pack 1021 shown in FIG. 52 in which a secondary battery 1022 and a bimetal switch 1023 are connected in series. If the bimetal switch 1023 is heated due to the heat generation of the secondary battery 1022 or the heat generation of the bimetal switch 1023 itself resulting from overcharging, for example, in the case of the breakdown of a charger 1026 connected with external connection terminals 1024, 1025, this protection circuit cuts off a charge current by turning the bimetal switch 1023 off to protect the secondary battery 1022.
Another known protection circuit is such as in a battery pack 1031 shown in FIG. 53 in which a PTC (Positive Temperature Coefficient) element 1032 in the form of a thermistor that is turned off in the case of exceeding a specified temperature is used, a secondary battery 1033 and the PTC element 1032 are connected in series. If the PTC element 1032 is heated due to the heat generation of the secondary battery 1032 or the heat generation of the PTC element 1033 itself resulting from overcharging, for example, in the case of the breakdown of a charger 1036 connected with external connection terminals 1034, 1035, this protection circuit cuts off a charge current by turning the PTC element 1032 off to protect the secondary battery 1032.
FIG. 54 is a circuit diagram showing another construction of a battery pack according to background art. A battery pack 1141 shown in FIG. 54 is a battery pack used in an electric apparatus, in which a large load current, e.g. 100 A (1 kW) flows such as an electric tool, an electric automobile or a robot. The battery pack 1141 is provided with a protection circuit 1142 and secondary batteries 1143 to 1146. A plurality of secondary batteries 1143 to 1146 are similar to the secondary battery 1003 in the battery pack 1001 and are connected in series to increase an output power.
The protection circuit 1142 is provided with external connection terminals 1147, 1148, FETs 1149, 1150, reference voltage generators 1151 to 1159, comparators 1160 to 1168, AND gates 1169, 1170, a resistor 1171 and a logic circuit 1172.
The external connection terminals 1147, 1148 are connection terminals similar to the external connection terminals 1004, 1005 in the battery pack 1001. The external connection terminal 1147, the secondary batteries 1143 to 1146, the FET 1149, the FET 1150 and the external connection terminal 1148 are connected in series.
The FET 1149 is used as a switch for protection from excessive discharge for cutting off discharge currents similar to the FET 1006 in the battery pack 1001 if discharge currents of the secondary batteries 1143 to 1146 become excessive, whereas the FET 1150 is used as a switch for protection from overcharging for cutting off charge currents similar to the FET 1007 in the battery pack 1001 if the secondary batteries 1143 to 1146 are overcharged.
The comparators 1160, 1162, 1164 and 1166 are for detecting the overcharging of the secondary batteries 1143, 1144, 1145 and 1146, and the comparators 1161, 1163, 1165 and 1167 are for detecting the excessive discharge of the secondary batteries 1143, 1144, 1145 and 1146.
Output voltages of the secondary batteries 1143, 1144, 1145 and 1146 and reference voltages outputted from the reference voltage generators 1151, 1152, 1153 and 1154 are compared by the comparators 1160, 1162, 1164 and 1166, and signals representing the comparison results are outputted to the AND gate 1169. If the output voltages of the secondary batteries 1143, 1144, 1145 and 1146 exceed the respective reference voltages, overcharging is detected and a low-level output voltage of the AND gate 1169 is outputted to the logic circuit 1172, which in turn turns the FET 1150 off to protect the secondary batteries 1143, 1144, 1145 and 1146 from overcharging.
Similarly, output voltages of the secondary batteries 1143, 1144, 1145 and 1146 and reference voltages outputted from the reference voltage generators 1155, 1156, 1157 and 1158 are compared by the comparators 1161, 1163, 1165 and 1167, and signals representing the comparison results are outputted to the AND gate 1170. If the output voltages of the secondary batteries 1143, 1144, 1145 and 1146 fall below the respective reference voltages, excessive discharge is detected and a high-level output voltage of the AND gate 1169 is outputted to the logic circuit 1172, which in turn turns the FET 1149 off to protect the secondary batteries 1143, 1144, 1145 and 1146 from excessive discharge.
Further, a connection point of the FETs 1149 and 1150 is connected to a plus terminal of the comparator 1168 via the resistor 1171, and a reference voltage outputted from the reference voltage generator 1159 is applied to a minus terminal of the comparator 1168. Thus, the discharge currents from the secondary batteries 1143 to 1146 flow through the FET 1149 and a voltage drop caused by the on-resistance of the FET 1149 is applied to the plus terminal of the comparator 1168 via the resistor 1171. The reference voltage generator 1159 is set to generate a voltage corresponding to a voltage drop caused by the on-resistance of the FET 1149, for example, if a maximum discharge current within such a range as not to cause the characteristic degradation of the secondary batteries 1143 to 1146 flows through the FET 1149.
Upon the flow of excessive discharge currents from the secondary batteries 1143 to 1146, the comparator 1168 detects an increase in the voltage drop in the FET 1149 and outputs a detection signal representing overcurrent discharge to the logic circuit 1172.
Upon the output of the detection signal representing overcurrent discharge from the comparator 1168, the logic circuit 1172 turns the FET 1149 off to stop the discharge of the secondary batteries 1143 to 1146. In this way, the protection circuit 1142 protects the secondary batteries 1143 to 1146 from excessive charging and discharging and overcurrent discharge.
However, the protection circuit 1002 shown in FIG. 51 cannot cut off the discharge current and the charge current having different directions of current flows by one FET since the FET includes a parasitic diode, and it has been necessary to provide the FET 1006 for cutting off the discharge current and the FET 1007 for cutting off the charge current. Further, the reference voltage generator 1008 and the comparator 1010 are necessary to detect overcharging; the reference voltage generator 1009, the comparator 1111, and the resistor 1112 are necessary to detect the excessive discharge current; and the logic circuit 1013 is necessary to turn the two FETS 1006, 1007 on and off in accordance with the output signals of the comparators 1010, 1111. This has caused inconvenience of increasing the circuit size of the protection circuit 1002.
Particularly if it is necessary to cause a large load current to flow as in the battery pack 1141 shown in FIG. 54, transistors with a large current rating need to be used as the FETs 1149, 1150, which has caused inconvenience of enlarging the FETs 1149, 1150 and increasing costs. Further, if the output voltage is increased by connecting the secondary batteries in series, the withstand voltages of the FETs 1149, 1150 need to be increased. However, since the transistors having high withstand voltages tend to increase the on-resistances, there has been inconvenience of increasing power losses in the FETs 1149, 1150.
Further, in a construction for protecting the secondary battery from overcharging by connecting a temperature switch, which operates with temperature, such as a bimetal switch or a PTC element with the secondary battery in series as shown in FIGS. 52 and 53, accuracy in detecting overcharging is low. Thus, if the secondary battery continues to be charged with such a charge current as not to suddenly increase temperature as in the case where a battery pack is charged, for example, using a charger having poor accuracy in controlling the charging voltage, the secondary battery is overcharged without the temperature switch operating, thereby causing inconvenience of degrading the characteristics of the secondary battery and leading to a likelihood of expanding or deforming the battery.
Patent Document 1: Japanese Unexamined Patent Publication No. H04-75430
Patent Document 2: Japanese Unexamined Patent Publication No. H11-262270